1. Field of Invention
The invention relates to signal processing, and more particularly to reduction of the quanitzation noise that is introduced in dial-up communication systems that perform multiple quantizations of analog signals.
2. Description of Related Art
Communication devices such as private branch exchanges (PBXs), automatic call distributors (ACDs) and modem banks are often connected to their local telephone offices by digital carrier systems, most commonly T1 lines. If the devices are connected to analog switches, such as the still-common commercial 1ESS, then calls between other telephone offices (that are connected to the local office by digital carriers) and the above mentioned devices must have their signals converted from digital to analog and back to digital again, to ensure compatible connection. Such signals are said to be double quantized.
However, that chain of processing raises the problem that if no other compensation is done, these signals have a signal to noise ratio approximately 3 db worse than signals that are digitized only once (single quantized). The extra, second quantization noise doesn't cause severe problems for voice signals (although degradation occurs), but often causes 28.8 Kbps and faster modems to have to fall back to a lower data rate.